Insulating Substrate and Manufacturing Method Therefor, and Multilayer Wiring Board and Manufacturing Method Therefor

ABSTRACT

There is provided a dimensionally accurate insulating substrate in which plane direction-wise shrinkage is practically zero and shrinkage variations are small. The insulating substrate includes a laminated body composed of at least two kinds of insulating layers made of crystallizable glass ceramics. The crystallization temperature of crystallizable glass contained in the first insulating layer is lower than the softening point of crystallizable glass contained in the second insulating layer. The difference in thermal expansion coefficient between the first and second insulating layers is preferably 2×10 −6 /° C. or below.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an insulating substrate, which issuitable for use as a circuit board, and a manufacturing method for theinsulating substrate, and a multilayer wiring board and a manufacturingmethod for the multilayer wiring board.

2. Description of the Related Art

While multilayer wiring boards using ceramics as their insulatingsubstrates have conventionally been in wide use, in keeping with therecent demand for multilayer wiring boards provided with variousfunctions, multilayer wiring boards fabricated by the use of acombination of ceramics materials of different types have been proposedto date. For example, there are known a multilayer wiring board thatoffers high strength by using a combination of a low-strength ceramicinsulating layer and a high-strength ceramic insulating layer, and amultilayer wiring board that allows incorporation of a high-capacitancecapacitor by using a laminated body composed of low-dielectric constantinsulating layers having sandwiched therebetween a high-dielectricconstant insulating layer.

In such a multilayer wiring board, to prevent generation of cracks anddelamination in ceramics materials, in general, the selection andcontrol of the characteristics of insulating-layer materials are madecarefully to make the ceramic insulating layers of different typesuniform in terms of firing shrinkage and thermal expansion coefficient.

Moreover, in order to carry out a reduction of costs for manufacturing amultilayer wiring board and enhancement of the dimensional accuracy ofan electrode formed on a multilayer wiring board (insulating substrate)or highly accurate mounting of constituent components, it has recentlybeen demanded that the firing shrinkage of an insulating substrateshould be reduced in the plane direction (X-Y direction) thereof, andwarpage and deformation resulting from shrinking-behavior variationsoccurring in the course of firing should be suppressed. Unfortunately,multilayer wiring boards of a related art, however, failed to meet sucha demand.

In order to satisfy such a requirement, the following methods have beendeveloped in recent years. Namely, on the one hand, in accordance with aso-called pressure-firing technique, an unfired laminated body composedof insulating green sheets (inclusive of a plurality of sheets to beformed into insulating layers of different types) is fired while beingpressurized through an Al₂O₃ sintered plate so as to increase the degreeof firing shrinking in the thickness direction (Z direction) thereof.Thereby, a wiring board having a desired insulating substrate isfabricated. On the other hand, on the surface of a green-sheet laminatedbody is disposed an unfired ceramic layer which will not be sintered ata temperature at which the laminated body is burned, so that thegreen-sheet laminated body is arrested by the unfired ceramic layer.Thereby, the shrinking of the laminated body is so controlled that itoccurs only in the thickness direction thereof. Afterwards, the unfiredceramic layer is removed (for example, refer to U.S. Pat. No.2,554,415). In either method, a conductor paste for forming a wiringpattern is applied to the surface of the green sheet, or is chargedinside the green sheet. Then, by firing the laminated body, aninsulating substrate and a wiring conductor layer can be obtained at thesame time (co-firing).

However, these methods have encountered the following problems. Namely,the former adopting the pressure-firing technique necessitates not onlya warpage-free Al₂O₃ sintered plate but also specially-devisedpressurizing means, whereas the latter exploiting the arresting actionof the unfired ceramic layer (restrictive firing) necessitates a stepfor removing the unfired ceramic layer after the firing process is over,which is liable to result in an undesirable increase in the number ofmanufacturing process steps.

In light of the foregoing, the following circuit board manufacturingmethod has been proposed to date (for example, refer to JapaneseUnexamined Patent Publication JP-A 2002-261443). Namely, in firing alaminated body composed of two kinds of insulating green sheets havingdifferent firing shrinking starting temperatures at one time, theirfiring shrinking starting temperatures are so controlled that, at thepoint in time when one green sheet having a higher firing shrinkingstarting temperature begins to shrink, the other green sheet having alower firing shrinking starting temperature has already arrived at 90%or more of a predetermined final firing volume shrinking amount. Thismakes it possible to suppress dimensional changes in a resulting circuitboard.

In the circuit board manufacturing method disclosed in JP-A 2002-261443,in firing a laminated body composed of two kinds of ceramic moldingshaving different firing shrinking starting temperatures at one time, itis necessary to control their firing shrinking starting temperatures insuch a manner that, at the point in time when one insulating layerhaving a higher firing shrinking starting temperature begins to shrink,the other insulating layer having a lower firing shrinking startingtemperature has already arrived at 90% or more of a predetermined finalfiring volume shrinking amount. This makes it possible to suppressdimensional changes in a resulting circuit board. However, on thenegative side, this method pays heed only to the control of firingshrinking starting temperatures. This could give rise to problems suchas occurrence of appreciable shrinking-behavior variations underrestriction and a failure of ensuring sufficiently low shrinkage.

According to the manufacturing method disclosed in JP-A 2002-261443,dimensional-change control can be exercised with ease. However, onlywith this advantage, it is difficult to ensure that the planedirection-wise shrinkage of the substrate becomes practically zero, orundesirable shrinkage variations are inevitable. Furthermore, in formingthe green sheet having a higher firing shrinking starting temperatureand the green sheet having a lower firing shrinking startingtemperature, while the characteristics of the sheet materials can bechanged according to purposes, it is necessary to make adjustment to theshrinking behaviors of the sheets individually. This imposes limitationson material design and thus the characteristics of materials cannot bechanged greatly. Accordingly, in order to attain improved capabilitymore effectively, another insulating green sheet which is free fromshrinking-behavior limitations needs to be prepared for use. Moreover,in a wiring conductor layer obtained by restrictive firing, its planedirection-wise shrinkage is restrained as with the case of thesubstrate. It is thus inevitable that most of the total necessary firingshrinkage of the wiring conductor layer will be compensated for by thethickness direction-wise shrinkage thereof. As a result, asperities tendto appear at the interface between the wiring conductor layer and theinsulating layer, in consequence whereof there results an increasedconductor resistance and poor interface conductivity. This leads up toelectrical characteristic degradation in a resulting insulator layerwith a built-in capacitor or the like function.

Also proposed is the following method (for example, refer to JP-A2002-290037). Namely, in firing a laminated body composed of two kindsof ceramic moldings having different firing shrinking startingtemperatures and a wiring layer at one time, the volume shrinkage andshrinking starting temperature of a conductor layer is so controlled asto prevent cracks and delamination from occurring in the vicinity of theinterface of the conductor layer.

However, in recent years, circuit boards have been becoming increasinglylower in profile and yet higher in performance. As a natural consequenceof this trend, slimness has been sought after in a ceramic molding foruse therein, and correspondingly the volumetric ratio of a conductorlayer present in the ceramic molding to the ceramic molding has beenincreasing. In the end, the ceramic molding and the conductor layer havebeen prone to vary in firing shrinking behavior, which leads up toappreciable warpage. This problem was taken no notice of inJP-A2002-261443 and JP-A 2002-290037.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a dimensionallyaccurate insulating substrate in which plane direction-wise shrinkage isreduced to practically zero and shrinkage variations can be decreased,as well as to provide a method for manufacturing the insulatingsubstrate.

It is another object of the invention to provide a dimensionallyaccurate multilayer wiring board having excellent electricalcharacteristics that is fabricated by firing together a plurality ofstacked insulating layers that exhibit different firing shrinkage curves(shrinking behaviors) in a manner so as to insure that the insulatinglayers of different types are inhibited from shrinking in a planedirection by each other, as well as to provide a method formanufacturing the multilayer wiring board.

It is yet another object of the invention to provide a multilayer wiringboard that is small in thickness and nevertheless suffer little fromwarpage.

The invention provides an insulating substrate comprising:

a laminated body including at least two kinds of glass-ceramicsinsulating layers,

wherein a crystallization temperature of crystallizable glass containedin a first insulating layer of the at least two kinds of glass-ceramicsinsulating layers is lower than a softening point of crystallizableglass contained in a second insulating layer thereof.

According to the invention, the insulating substrate comprises alaminated body composed of at least two kinds of glass-ceramicsinsulating layers. In this construction, by setting the crystallizationtemperature of the crystallizable glass contained in the firstinsulating layer to be lower than the softening point of thecrystallizable glass contained in the second insulating layer, it ispossible to attain high dimensional accuracy, to ensure that planedirection-wise shrinkage becomes practically zero, and to decreaseshrinkage variations.

In the invention, it is preferable that a difference in thermalexpansion coefficient between the first insulating layer and the secondinsulating layer is given by: 2×10⁻⁶/C or below.

According to the invention, by setting the difference in thermalexpansion coefficient between the first insulating layer and the secondinsulating layer at 2×10⁻⁶/° C. or below in particular, it is possibleto prevent more effectively generation of cracks and delamination in themultilayer board.

In the invention, it is preferable that the first and the secondinsulating layers each have a crystallizable glass content of 30% bymass or above.

According to the invention, in a case where the first and the secondinsulating layers each contain crystallizable glass in an amount of 30%by mass or above, still stabler sinterability and adherability can beattained.

In the invention, it is preferable that the first and second insulatinglayers each have a residual glass content of 10% by volume or below.

It is desirable that each of the first and second insulating layersshould have a residual glass content of 10% by volume or below in termsof plane direction (X-Y direction)-wise shrinking restriction effect,substrate strength in bending, and dielectric loss.

In the invention, it is preferable that the crystallizable glasscontained in the first and second insulating layers forms at least oneselected from the group consisting of diopside, hardestnite, celsian,cordierite, anorthite, gahnite, willemite, spinel, mullite, forsteriteand suanite.

According to the invention, in a case where the crystallizable glasscontained in the first and second insulating layers forms at least oneselected from the group consisting of diopsider hardestnite, celsian,cordierite, anorthite, gahnite, willemite, spinel, mullite, forsterite,and suanite, improved dielectric characteristics or strength can beattained.

The invention provides a multilayer wiring board comprising:

the insulating substrate mentioned above; and

wiring layers formed on surfaces of the insulating substrate and in aninterior of the insulating substrate,

wherein the insulating substrate and the wiring layers are formed byco-firing,

and wherein the wiring layer has a cross section whose void ratio isreduced to 5% or below.

According to the invention, in the multilayer wiring board, the ratio ofvoid in the cross section of the wiring layer is kept at 5% or below. Inthis case, even if the insulating substrate includes at least two kindsof insulating layers having different firing shrinking startingtemperatures, the conductor resistance becomes sufficiently low and yetthe interface conductivity becomes sufficiently high.

In other words, in the multilayer wiring board of the invention, theproportion of void to the area of the cross section of the wiring layeris as small as 5% or below. Therefore, the insulating layers havingdifferent shrinking starting temperatures are allowed to exert anarresting action on each other at their intimate-contact portions in thecourse of firing. In consequence, even in a case where plane direction(X-Y direction) wise firing shrinking is restricted, it is possible todecrease the conductor resistance and yet increase the interfaceconductivity. For example, if the proportion of void to the area of thecross section of the wiring layer exceeds 5%, as has been attested bythe experimental example that will be explained later (test sample Nos.21, 22), the sheet resistance will take on a value as high as 1.6mΩ/square or above. This causes the electrical characteristics of themultilayer wiring board to deteriorate.

That is, in performing restrictive firing, it is desirable to set theproportion of void to the area of the cross section of the wiring layerat 5% or below. By virtue of such a small voidage, it is possible tomake the wiring layer as a whole less defective. Moreover, generation ofasperities can be prevented from occurring at the interface between thewiring layer and the insulating layer, wherefore the wiring layer can beformed with high dimensional accuracy, low conductor resistance, andhigh interface conductivity. Hence, according to the invention, amultilayer wiring board incorporating excellent capability can berealized without the necessity of imposing strict limitations onmaterial design.

In the invention, it is preferable that the wiring layer predominantlycontains at least one selected from the group consisting of Au, Ag, Cu,Pd, and Pt.

The invention provides a method for manufacturing an insulatingsubstrate, comprising the steps of:

preparing a laminated body composed of first and second insulatingsheets containing crystallizable glass powder and ceramic powder,wherein a crystallization temperature of the crystallizable glass powdercontained in the first insulating sheet is lower than a softening pointof the crystallizable glass powder contained in the second insulatingsheet; and

carrying out co-firing of the laminated body to fabricate an insulatingsubstrate.

According to the invention, the first and second insulating sheets canexert shrinking restriction effects on each other with stability. Thismakes it possible to provide a dimensionally accurate insulatingsubstrate in which firing shrinkage variations can be decreased and theshrinkage can be reduced to practically zero.

In the invention, it is preferable that the first and second insulatingsheets each have a crystallizable glass powder content of 30% by mass orabove.

In a case where the first and second insulating sheets each have acrystallizable glass powder content of 30% by mass or above inparticular, still stabler sinterability and adherability can beattained.

In the invention, it is preferable that the crystallizable glasscontained in the first and second insulating sheets forms at least oneselected from the group consisting of diopside, hardestnite, celsian,cordierite, anorthite, gahnite, willemite, spinel, mullite, forsterite,and suanite.

In the invention, it is preferable that the crystallizable glass powdercontained in the first insulating sheet comprises: 5 to 20% by mass ofSiO₂; 40 to 50% by mass of MgO; 10 to 30% by mass of B₂O₃; and 0 to 30%by mass of at least one selected from the group consisting of CaO,Al₂O₃, SrO, ZnO, TiO₂, Na₂O, BaO, SnO₂, P₂O₅, ZrO₂, and Li₂O, whereasthe crystallizable glass powder contained in the second insulating sheetcomprises: 30 to 50% by mass of SiO₂; 10 to 25% by mass of MgO; and 25to 55% by mass of at least one selected from the group consisting ofB₂O₃, CaO, Al₂O₃, SrO, ZnO, TiO₂, Na₂O, SnO₂, P₂O₅, ZrO₂, and Li₂O.

According to the invention, in a case where the crystallizable glasspowder contained in the first and second insulating sheets forms atleast one selected from the group consisting of diopside, hardestnite,celsian, cordierite, anorthite, gahnite, willemite, spinel, mullite,forsterite, and suanite, improved dielectric characteristics or strengthcan be attained.

The invention provides a method for manufacturing a multilayer wiringboard comprising the steps of:

preparing a plurality of the first and second insulating sheetsmentioned above;

applying a conductor paste in a wiring pattern at least to surfaces ofpartial insulating sheets among a plurality of the insulating sheets toform a conductor layer, the conductor layer predominantly containing atleast one selected from the group consisting of Au, Ag, Cu, Pd, and Pt;

fabricating a laminated body by stacking a plurality of the insulatingsheets; and

sintering the laminated body.

In the invention, it is preferable that the conductor paste is preparedby blending an organic binder and a solvent into metal powder having atap bulk density of 15% or above based on a density (d²⁰) of a metalelement.

According to the invention, in order to adjust the proportion of void tothe area of the cross section of the wiring conductor layer to be 5% orbelow, the wiring conductor layer is formed of a conductor pasteprepared by blending suitable organic binder and solvent into metalpowder having a tap bulk density of 15% or above based on the density ofthe metal element.

In the invention, it is preferable that the second insulating sheet hasa firing shrinking starting temperature T₂ which is higher than both ashrinking ending temperature T₃ of the first insulating sheet and ashrinking ending temperature T₄ of the conductor layer.

In the invention, it is preferable that B₁/B₂ is 0.90 or above, in whichB₁ represents a volume content of the inorganic compound composed of thecrystallizable glass powder and the ceramic powder contained in thefirst insulating sheet, and B₂ represents a volume content of theinorganic compound composed of the crystallizable glass powder and theceramic powder contained in the second insulating sheet.

In the invention, it is preferable that B₃/B₂ is 0.90 or above, in whichB₃ represents a volume content of the metal compound contained in theconductor layer.

The invention has been devised based on the knowledge that, when thefiring shrinking ending temperature of the second insulating sheet andthe firing shrinking ending temperature of the conductor layer are lowerthan the shrinking starting temperature of the first insulating sheet,it is possible to reduce the plane direction (X-Y direction)-wise firingshrinking of the multilayer wiring board to a minimum and therebyminimize warpage occurring in the multilayer wiring board.

Specifically, during the time interval between starting and ending offiring shrinking in the second insulating sheet and the conductor greensheet, X-Y direction-wise firing shrinking is satisfactorily restrictedby the other ceramic green sheet in a non-shrinking state. Then, by thetime the first insulating sheet begins to shrink, the firing shrinkingof the second insulating sheet and the conductor green sheet is over,and thus they in turn act to restrict X-Y direction-wise firingshrinking satisfactorily.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a schematic sectional view showing a multilayer wiring boardaccording to a first embodiment of the invention;

FIG. 2 is a schematic sectional view showing a multilayer wiring boardaccording to a second embodiment of the invention; and

FIG. 3 is a view of assistance in explaining shrinking behaviors asobserved in the multilayer wiring board shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to the drawings, preferred embodiments of the inventionare described below.

First Embodiment

FIG. 1 is a schematic sectional view showing a multilayer wiring boardaccording to a first embodiment of the invention. In FIG. 1, themultilayer wiring board 10 is composed of: an insulating substrate 1composed of a laminated body constituted by stacking together insulatinglayers 1 a through 1 g; surface conductor layers 2 formed on the top andback surfaces of the insulating substrate 1; inner conductor layers 3formed within the insulating substrate 1; and a via-hole conductor 4 forproviding connection among the conductor layers.

In more detail, in the multilayer wiring board 10 of the inventionhaving such a constitution, the insulating substrate 1 is formed bystacking a plurality of insulating layers 1 a through 1 g on top of oneanother thereby to assume a lamination structure. The inner conductorlayer 3 is interposed between the adjacent insulating layers. Thevia-hole conductor 4 is so formed as to extend penetratingly through atleast one of the insulating layers.

The insulating substrate 1 is composed of first and second insulatinglayers made of glass ceramics materials having different shrinkingstarting temperatures. In the multilayer wiring board 10 shown in FIG.1, of the insulating layers 1 a through 1 g, for example, the insulatinglayers 1 a and 1 g may be defined as the first insulating layer, whereasthe insulating layers 1 b through if may be defined as the secondinsulating layer. The crystallization temperature of crystallizableglass contained in the first insulating layer is lower than thesoftening point of crystallizable glass contained in the secondinsulating layer.

In more detail, in the invention, the insulating layers 1 a through 1 gconstituting the insulating substrate 1 are classified at least undertwo groups according to their firing shrinking starting temperatures.For example, the insulating layers 1 a, 1 g located on the top and backsurfaces of the insulating substrate 1 are made of the same material,and thus have the same firing shrinking starting temperature. Theinsulating layers 1 a and 1 g belong to a “first insulating layer”group. On the other hand, the insulating layers, 1 b through if locatedinside the insulating substrate 1 are made of a material which isdifferent from that used for the first insulating layer, and thus have afiring shrinking starting temperature which is different from that ofthe first insulating layer. The insulating layers 1 b through if belongto a “second insulating layer” group.

The classification of the insulating layers 1 a through 1 g into aplurality of groups is necessary to prevent the warpage and improve thedimensional accuracy by suppressing the shrinkage in the X-Y directionin firing the multilayer wiring board 10 (insulating substrate 1). Morespecifically, referring to FIG. 1, by employing materials having a highshrinking starting temperature for formation of the insulating layers 1b through 1 f located inside the insulating substrate 1, and materialshaving a low shrinking starting temperature for formation of the surfaceinsulating layers 1 a, 1 g, it is made possible to inhibit the mutualshrinkage of the insulating layers on interfaces between differentmaterials when these insulating layers are co-tired.

Although the above description deals with the case where the insulatinglayers 1 a through 1 g are classified under two groups: the firstinsulating layer group and the second insulating layer group, theinvention is not limited thereto as a matter of course. Depending uponwhat kind of capability the insulating substrate is given interiorly,the insulating layers 1 a through 1 g may be classified under three ormore groups according to physical properties such as relativepermittivity, bending strength, dielectric loss, thermal conductivity,volume density, and temperature coefficient. For example, in the exampleshown in FIG. 1, the insulating layers 1 c and 1 d located in the middleof the insulating substrate may be defined as a third insulating layer,so long as they are made of another material which differs in sinteringstarting temperature from the materials constituting the first andsecond insulating layers (a material such as that which has a stillhigher dielectric constant, for instance). Also in this case, it ispossible to impart a capacitor capability to the interior of theinsulating substrate. The way of classifying the insulating layers 1 athrough 1 g under the first, second and third (or more) insulating layergroups is determined appropriately in consideration of a targetcapability to be incorporated.

Now, a description will be given below as to the thermal characteristicsof the crystallizable glass materials contained in the first insulatinglayer 1 a, 1 g and the second insulating layer 1 b through 1 f.

It is preferable that the insulating layers 1 a through 1 g are eachmade of glass ceramics containing crystallizable glass from theviewpoint of inhibiting plane direction-wise shrinking. For example, theselection of glass ceramic materials for forming the insulating layers 1a through 1 g is made in a manner so as to ensure that thecrystallization temperature of glass to be contained in the firstinsulating layer (the insulating layers 1 a, 1 g) is lower than thesoftening point of glass to be contained in the second insulating layer(the insulating layers 1 b through 1 f). This helps reduce planedirection-wise shrinking to practically zero. Given that thecrystallization temperature of the glass contained in the firstinsulating layer (1 a, 1 g) is lower than the softening point of theglass contained in the second insulating layer (1 b through 1 f), sincethe first insulating layer begins to shrink at a relatively lowtemperature, it follows that, during firing, at the point in time whenthe second insulating layer (1 b through 1 f) begins to shrink, thefiring shrinking of the first insulating layer (1 a, 1 g) is nearlyover, specifically, for example, the first insulating layer (1 a, 1 g)has already arrived at 97% or above, especially 98% or above, moreespecially 99% or above, of a predetermined final firing volumeshrinking amount. This means that the second insulating layer isinhibited from shrinking during the interval when the first insulatinglayer is shrinking, whereas the first insulating layer is inhibited fromshrinking during the interval when the second insulating layer isshrinking. As a result, the plane direction (X-Y direction)-wiseshrinking of the first insulating layer and the plane direction (X-Ydirection)-wise shrinking of the second insulating layer are mutuallyinhibited, whereby making it possible to reduce the shrinking to nearlyzero while decreasing undesirable shrinkage variations.

In the invention, in order to achieve the above-described mutual planedirection-wise shrinking inhibition more effectively, it is desirablethat the crystallization temperature of the glass contained in the firstinsulating layer (1 a, 1 g) should be 10° C. or more lower than thesoftening point of the glass contained in the second insulating layer (1b through 1 f). Note that, in a case where the insulating layers 1 athrough 1 g are classified under three (or more) insulating layergroups, from a delamination or warpage standpoint, the selection ofmaterials for forming the insulating layers belonging to the thirdinsulating layer group (and the ones belonging to another group/groups)is preferably made in a manner so as to ensure that these insulatinglayers are similar in shrinking behavior to one of the first and secondinsulating layers.

As the glass material of the insulating layers 1 a through 1 g, it isdesirable to use crystallizable glass which is capable of precipitating,at the time of firing, at least one selected from the group consistingof diopside, hardestnite, celsian, cordierite, anorthite, gahnite,willemite, spinel, mullite, forsterite, and suanite. In a case where theinsulating layers 1 a through 1 g assume such a crystalline phase asmentioned just above, enhancement of characteristics, for example anexcellent dielectric characteristic and high mechanical strength can beachieved. From a dielectric characteristic standpoint, crystallizableglass which is capable of precipitating diopside, hardestnite, celsian,willemite, or forsterite is particularly desirable. From a strengthstandpoint, crystallizable glass which is capable of precipitatingdiopside, celsian, cordierite, or anorthite is particularly desirable.With consideration given to both a dielectric characteristic andstrength, crystallizable glass which is capable of precipitatingdiopside or celsian is particularly desirable.

Moreover, in the invention, given that the insulating layers 1 a through1 g for constituting the insulating substrate 1 are classified under atleast two groups that are different from each other in terms ofmaterial, the selection of materials is preferably made in a manner soas to ensure that the difference in thermal expansion coefficient (in arange of from ambient temperature to 900° C.) between the firstinsulating layer and the second insulating layer is not greater than2×10⁻⁶/° C., especially 1×10⁻⁶/° C. If the thermal expansion coefficientdifference exceeds 2×10⁻⁶/° C., as the highest firing temperature growscolder and colder, a difference in thermal shrinkage arises, inconsequence whereof there results generation of cracks and delaminationat the interface between the first and second insulating layers. Bysetting the thermal expansion coefficient difference to be 2×10⁻⁶/° C.or lower, it is possible to prevent generation of cracks anddelamination that is ascribable to a thermal expansion (thermalshrinkage) difference effectively in the course of cooling subsequent tofiring.

The above-described difference in thermal expansion coefficient betweenthe layers can be controlled in the following manner. Namely, inaddition to the crystallizable glass constituent, a filler constituentis contained in the insulating layers 1 a through 1 g, and then theamounts of the constituents are adjusted properly to serve the purpose.

The specific examples of ceramics serving as the filler contained in thefirst and second insulating layers include: Al₂O₃; SiO₂; MgTiO₃; CaZrO₃;CaTiO₃; Mg₂SiO₄; BaTi₄O₉; ZrTiO₄; SrTiO₃; BaTiO₃; TiO₂; AlN; and SiN.Among them, Al₂O₃, MgTiO₃, CaZrO₃, CaTiO₃, Mg₂SiO₄, and BaTi₄O₉ areparticularly desirable from a dielectric characteristic standpoint;Al₂O₃, AlN, and SiN are particularly desirable from a strengthstandpoint; and Al₂O₃ is particularly desirable from the standpoint ofboth a dielectric characteristic and strength.

As for the above-described crystallizable glass and filler constituents,in the invention, with consideration given to sinterability andadherability between the layers, it is preferable that the insulatinglayers 1 a through 1 g each have a crystallizable glass content of 30%by mass or above, especially 40 to 90% by mass, and more especially 50to 80% by mass. This means that the content of the filler is 70% by massor below. In a case where the content of the crystallizable glass isequal to or greater than 30% by mass, the proportion of the glass in theinsulating layer is high enough to exploit the viscous flowability ofthe glass, which results in an advantage in attaining stablesinterability and adherability.

According to the invention, it is preferable that the first and secondinsulating layers each have a crystallizable glass content of 30% bymass or above, especially 40 to 90% by mass, and more especially 50 to80% by mass. It is also preferable that the first and second insulatinglayers each have a residual crystallizable glass content of 10% byvolume or below, especially 5% by volume or below, and more especially2% by volume or below in terms of X-Y direction-wise shrinkingrestriction effect, substrate strength in bending, and dielectric loss.Note that the amount of the glass remaining may be determined by theRietvelt analyzing method using an XRD diffraction pattern. In order todetermine a fixed amount of the glass, a specimen substance and ZnO(standard test sample) are admixed in a predetermined ratio, and aprogrammed data on all of the crystalline phases that will appear in thespecimen substance and the standard test sample ZnO is analyzed.

As stated hereinabove, the insulating layers 1 a through 1 g forconstituting the insulating substrate 1 to be included in the multilayerwiring board 10 of the invention can be formed of a glass ceramicsmaterial composed of crystallizable glass and ceramics. Accordingly, afiring process can be achieved at a temperature of 1000° C. or below.This makes it possible to use a low-resistance conductor element, suchas Cu, Ag, and Al, to form a wiring layer. Moreover, by virtue of theadvantage of providing a lower dielectric constant, the invention issuitable for high-speed transmission. Thus, according to the invention,a multilayer wiring board which is excellent in dimensional accuracy canbe produced with high reproducibility.

Note that the first and second insulating layers can be materiallydesigned according to purposes. For example, the materialcharacteristics such as relative permittivity, bending strength,dielectric loss, thermal conductivity, bulk density, and temperaturecoefficient can be changed on an as needed basis.

Moreover, in FIG. 1, the first insulating layers (A) and the secondinsulating layers (B) are arranged in the lamination order of ABBBBBA.However, the invention is not limited thereto. For example, thelamination order may be any of ABABABA, AAABAAA, AABBBAA, AABABAA,AABBAAA, ABAAAAA, ABAAABA, ABBABBA, AABAAAA, ABBAAAA, ABBBAAA, andABBBBAA. Otherwise, in these lamination orders, A and B may be arrangedin the place of each other.

Incidentally, as stated just above, in the case of using various kindsof materials having different sintering starting temperatures forforming the insulating layers 1 a through 1 g, when the insulatinglayers 1 a through 1 g are co-fired, problems may arise with theproduction of the multilayer wiring board 10. For example, the firstinsulating layer and the second insulating layer exhibit differentfiring shrinking behaviors. This leads to poor dimensional accuracy andthe risk of damage such as a crack inside the insulating substrate 1.Furthermore, the wiring layers (the surface conductor layer 2, the innerconductor layer 3, and the via-hole conductor 4) cannot be disposed onthe surface and in the interior of the insulating substrate 1 with highaccuracy, which makes fine wiring or the like process difficult. In viewof the foregoing, in the invention, the wiring layers, namely thesurface conductor layer 2, the inner conductor layer 3, and the via-holeconductor 4 are so designed that the ratio of void is kept as low as 5%or below, especially 2.5% or below, when viewed in cross section.

In other words, in the invention, by virtue of a low void ratio in thecross section of the wiring layer, the first and second insulatinglayers having different shrinking starting temperatures exert anarresting action on each other at their intimate-contact portions in thecourse of firing. As a result, in a case where firing shrinking isrestricted in the plane direction (X-Y direction), it is possible todecrease plane direction-wise shrinkage and thus shrinkage variations.Moreover, since the void ratio is kept as low as possible, it followsthat the conductor resistance can be decreased and yet the interfaceconductivity can be increased in the wiring layers (the surfaceconductor layer 2, the inner conductor layer 3, and the via-holeconductor 4). This makes it possible to enhance the capabilityincorporated in the multilayer wiring board 10.

Next, a multilayer wiring board manufacturing method of the inventionwill be explained in detail.

At the outset, powder of raw materials for forming the first and secondsheets, specifically, crystallizable glass powder and ceramic powderserving as filler are prepared for use. In other words, in order toobtain ceramic green sheets to be formed into the insulating layers 1 athrough 1 g, the crystallizable glass in powder form and the filler aremixed together at the above-described mixture ratio. In preparing such apowder admixture, as has already been explained, the selection of thecrystallizable glass used to form the green sheet to be formed into thefirst insulating layer (1 a, 1 g) is preferably made in a manner so asto ensure that the crystallization temperature T_(c) of thecrystallizable glass is lower than (especially, 10° C. or more lowerthan) the softening point T_(g) of the glass to be contained in thesecond insulating layer (1 b through 1 f). As the crystallizable glasspowder, it is desirable to use a substance which is capable ofprecipitating at least one selected from the group consisting ofdiopside, hardestnite, celsian, cordierite, anorthite, gahnite,willemite, spinel, mullite, forsterite, and suanite, in the wake offiring from the standpoint of dielectric characteristics or strength.

On the other hand, as the ceramic powder, it is desirable to use atleast one selected from the group consisting of Al₂O₃ powder; SiO₂powder; MgTiO₃ powder; CaZrO₃ powder; CaTiO₃ powder; Mg₂SiO₄ powder;BaTi₄O₉ powder; ZrTiO₄ powder; SrTiO₃ powder; BaTiO₃ powder; TiO₂powder; AlN powder; and Si₃N₄ powder, from the standpoint of dielectriccharacteristics or strength.

Here, it is preferable that the first and second insulating sheets eachhave a crystallizable glass powder content of 30% by mass or above,especially 40 to 90% by mass, and more especially 50 to 80% by mass,from the standpoint of sinterability.

In more detail, it is preferable that the crystallizable glass powdercontained in the first insulating sheet comprises: 5 to 20% by mass ofSiO₂; 40 to 50% by mass of MgO; 10 to 30% by mass of B₂O₃; and 0 to 30%by mass of at least one selected from the group consisting of CaO,Al₂O₃, SrO, ZnO, TiO₂, Na₂O, BaO, SnO₂, P₂O₅, ZrO₂, and Li₂O. It ispreferable that the crystallizable glass powder contained in the secondinsulating sheet comprises: 30 to 50% by mass of SiO₂; 10 to 25% by massof MgO; and 25 to 55% by mass of at least one selected from the groupconsisting of B₂O₃, CaO, Al₂O₃, SrO, ZnO, TiO₂, Na₂O, SnO₂, P₂O₅, ZrO₂,and Li₂O.

Subsequently, with use of these powder materials, green sheets areformed as the first and second insulating sheets in the followingmanner. Firstly, a predetermined ceramic powder compound is mixed with avolatile organic binder which is caused to volatilize easily in thecourse of firing and an organic solvent, and, if necessary, aplasticizer, to form a slurry. The slurry is formed into a tape-likemolding by a known technique such as the lip coater method or the doctorblade method, and the molding is then cut up to obtain green sheets ofpredetermined dimension. Note that, in some cases, one of the insulatinglayers may be made in the form of a paste.

In other words, the selection of the crystallizable glass used to formthe green sheet to be formed into the first insulating layer (1 a, 1 g)is preferably made so as to ensure that the crystallization temperatureT_(c) of the crystallizable glass is lower than (especially, 10° C. ormore lower than) the softening point T_(g) of the glass to be containedin the second insulating layer (1 b through 1 f). Based on thiscondition, various changes are made to the composition (the kind and/oramount of the crystallizable glass and/or filler) of the powderadmixture. The powder admixtures of varying types thus obtained are eachadded with a volatile binder which is caused to volatilize in the courseof firing (for example, ethylcellulose or acrylic resin), a solvent (forexample, alcohol-base solvent such as isopropyl alcohol), and, ifnecessary, a plasticizer, to form a molding slurry or paste. The slurry(or paste) is molded into green sheets by a known molding technique suchas the lip coater method or doctor blade method, whereupon the greensheets for constituting the first insulating layer (1 a, 1 g) and thesecond insulating layer (1 b through 1 f) are prepared for use.

In the green sheets thus formed, depending upon which insulating layeris acquired, the surface conductor layer 2, the inner conductor layer 3,or the via-hole conductor 4 needs to be formed with use of a conductorpaste. Specifically, the conductor paste is applied in conformity withthe predetermined patterns of the surface conductor layer 2 and theinner conductor layer 3 by means of screen printing or the like method,or is charged in a through hole drilled by punching or the like processat a position corresponding to the via-hole conductor 4.

In order to obtain the conductor paste, powder of a metal material (forexample, a low-resistance element such as Cu, Ag, and Al) suitable forforming the surface conductor layer 2, the inner conductor layer 3, andthe via-hole conductor 4 (they will collectively be referred to simplyas “wiring layer”) is prepared first. Then, just as in the case offorming the green sheets for constituting the insulating layers,suitable organic binder and organic solvent are blended into the metalpowder. Note that, in the invention, it is essential to use metal powderhaving a tap bulk density of the order of 15% or above, especially 35%or above, based on the density (d²⁰) of the metal element (according tothe density measurement method prescribed in JIS R1628). The use of themetal powder having undergone substantial tap-bulk-density adjustmentmakes it possible to ensure that the ratio of void in the cross sectionof the wiring layer is kept at 5% or below, especially 2.5% or below. Inthis way, the insulating layers make intimate contact with one another,wherefore, as stated previously, the first and second insulating layersexert an arresting action on each other satisfactorily in the course offiring. This makes it possible to lessen firing shrinking, to avoidshrinkage variations, and to decrease the conductor resistance and thusenhance the interface conductivity in the wiring layer.

In other words, the metal powder employed in the invention possesses aconsiderably high tap bulk density. By using such a metal powdermaterial with highly dense metal powder particles in which little voidis present between the particles, it is possible to keep the ratio ofvoid in the wiring layer as low as 5% or below, especially 2.5% orbelow, when viewed in cross section.

In order to increase the tap bulk density of the metal powder to theaforementioned level, a large number of fine-grain particles arecontained therein, and then the metal powder is subjected to agitationin a ball mill. In some cases, a pressurization treatment is carried outunder reduced pressure. It is also effective to use flat-shaped powderparticles to increase the tap bulk density.

Alternatively, as the metal powder, an admixture of powder materials oftwo or more kinds is usable. In this case, the metal-element density(d²⁰) of the powder admixture can be calculated by adding together thedensities (d²⁰) of the individual metal elements contained in the powderadmixture on the basis of the mass ratio between the metal elements, andthereby the ratio of the tap bulk density to the metal-element densitycan be obtained by calculation. The above-described way of calculationis also true for the case where the metal powder particle has itssurface coated with glass, oxide, or the like substance to exerciseshrinking-behavior control.

In general, it is preferable that the conductor paste has a metal powdercontent of 80% by mass or above, especially, 85% by mass or above. Ifthe metal powder content is unduly low, the gap between the particleswill be increased in size due to entrained air or the flowing action ofthe conductor paste accompanied by application, which leads to anundesirable increase in the ratio of void in the cross section of thewiring conductor layer.

The green sheets to be formed into the first insulating layer (1 a, 1 g)and the second insulating layer (1 b through 1 f) thus obtained aresubjected to compression bonding to form a laminated body. The laminatedbody is, after undergoing a binder removal treatment, fired at a highertemperature than before, whereupon the multilayer wiring board 10 of theinvention is constructed.

In general, a firing process is performed at a temperature of 1000° C.or below, especially within a range of from 850 to 950° C. However, asstated above, the ratio of void in the conductor paste used to form thewiring layer is significantly reduced, thereby ensuring tight adherencebetween the first insulating layer (1 a, 1 g) and the second insulatinglayer (1 b through 1 f) at the interface therebetween. Therefore, thefirst and second insulating layers exert a satisfactory arresting actionon each other at the interface. As a result, the following advantagesare gained: the difference in firing shrinking between the first andsecond insulating layers can be decreased; the plane direction-wiseshrinkage can be inhibited; and shrinkage variations can be reduced.

Moreover, in the invention, in the green sheets formed in the abovestated manner, given that the shrinking ending temperature of the greensheet to be formed into the first insulating layer is T₃ and that theshrinking starting temperature of the green sheet to be formed into thesecond insulating layer is T₂, then, from the fact that thecrystallization temperature Tc of the crystallizable glass contained inthe first insulating layer (1 a, 1 g) is lower than the softening pointTg of the crystallizable glass contained in the second insulating layer(1 b through 1 f), the following relationship holds. Note that thetemperatures T₃, T₂, etc. can be measured by means of TMA (ThermoMechanical Analysis) or DTA (Differential Thermal Analysis).

T₃<Tc<Tg<T₂

As will be understood from the above expression representing thetemperature relationship, the green sheet to be formed into the secondinsulating layer (1 b through 1 f) is kept in a non-shrinking stateduring the interval when the green sheet to be formed into the firstinsulating layer (1 a, 1 g) is shrinking. Accordingly, the green sheetto be formed into the second insulating layer in a non-running stateacts to inhibit the plane direction-wise shrinking of the green sheet tobe formed into the first insulating layer. On the other hand, at thepoint in time when the green sheet to be formed into the secondinsulating layer begins to shrink in the course of sintering, theshrinking of the green sheet to be formed into the first insulatinglayer is nearly over. Accordingly, the green sheet to be formed into thefirst insulating layer in a non-running state acts to inhibit the planedirection-wise shrinking of the green sheet to be formed into the secondinsulating layer. As a result, the plane direction-wise firing shrinkingof the substrate as a whole obtained after firing can be restricted evenfurther. Moreover, since the crystallization temperature of the glasscontained in the green sheet to be formed into the first insulatinglayer is lower than the softening point of the glass powder contained inthe green sheet to be formed into the second insulating layer, itfollows that, at the point in time when the green sheet to be formedinto the second insulating layer begins to shrink, the shrinking of thegreen sheet to be formed into the first insulating layer is nearly overand thus it is brought into a crystallized state. This makes it possibleto provide a multilayer wiring board in which shrinkage variations aredecreased, the shrinkage is reduced to practically zero, and highdimensional accuracy is attained.

After all, a final firing process is performed at a temperature which ishigher than the shrinking starting temperature T₂ of the green sheet tobe formed into the second insulating layer (1 b through 1 f). In thisregard, the so-called multi-step firing process may be adopted. Namely,for example, in the first firing step, the firing temperature isadjusted to fall in between the shrinking ending temperature T₁ of thesheet to be formed into the first insulating layer and thecrystallization temperature Tc of the glass powder contained in thesheet to be formed into the first insulating layer, and, in the nextfiring step, the firing temperature is changed to be higher than theshrinking starting temperature T₂. This makes it possible achieve mutualshrinking inhibition between the first insulating layer-sheet and thesecond insulating layer-sheet more effectively.

Second Embodiment

FIG. 2 is a schematic sectional view showing a multilayer wiring boardaccording to a second embodiment of the invention. In FIG. 2, themultilayer wiring board 2 is composed of: an insulating substrate 11constituted by stacking together second insulating sheets 11 a through11 c which are ceramic green sheets and combined layers 12 a and 12 b;surface conductor layers 12 formed on the top and back surfaces of theinsulating substrate 11; inner conductor layers 13 formed within theinsulating substrate 11; and a via-hole conductor 14 for providingconnection among the conductor layers.

The combined layers for constituting the insulating substrate 11 arecomposed of first and second insulating sheets which are ceramic greensheets having different shrinking starting temperatures. Specifically,the firing shrinking starting temperature of the insulating sheets 12a-(1) 12 b-(1) is lower than those of the other insulating sheets 12a-(2), 12 b-(2), and 11 a through 11 c. Meanwhile, the firing shrinkingending temperature of the insulating sheets 12 a-(1), 12 b-(1) is lowerthan the firing shrinking starting temperatures of the insulating sheets12 a-(2), 12 b-(2), and 11 a through 11 c. Moreover, the firingshrinking ending temperature of the conductor green sheet used to formthe surface conductor layer 12 and the inner conductor layer 13 is lowerthan those of the insulating sheets 12 a-(2), 12 b-(2), and 11 a through11 c.

Now, a description will be given as to the outline of the firingshrinking behaviors of the two types of ceramic green sheets and theconductor green sheet with reference to FIG. 3 showing firing shrinkagecurves. According to FIG. 3, in the invention, what is essential isthat, given that the two first and second insulating sheets of differentfiring shrinking starting temperatures have firing shrinking startingtemperatures T₁ and T₂, respectively, then the relationship between T₁and T₂ is given by: T₁<T₂, and that, given that the first insulatingsheet and the conductor green sheet have firing shrinking endingtemperatures T₃ and T₄, respectively, then the relationship among T₃,T₄, and T₂ is given by: T₃<T2, and T₄<T₂.

In the multilayer wiring board shown in FIG. 2, for example, in theinsulating substrate 11, the insulating sheets 12 a-(1) and 12 b-(1)correspond to the first insulating sheet, the insulating sheets 12a-(2), 12 b-(2), and 11 a through 11 c correspond to the secondinsulating sheet, and the surface conductor layer 2 and the innerconductor layer 3 correspond to the conductor green sheet C which is aconductor layer.

According to the invention, what is essential is that, at the point intime when the second insulating sheet having a highest firing shrinkingstarting temperature begins to shrink in the course of firing, thefiring shrinking of the first insulating sheet and the conductor greensheet C should be over.

If the first insulating sheet and the conductor green sheet C are stillin a firing shrinking state when the second insulating sheet having ahighest firing shrinking starting temperature begins to shrink, thefirst insulating sheet, the second insulating sheet, and the conductorgreen sheet C will fail to exert a satisfactory arresting action on oneanother during the firing shrinking, in consequence whereof thereresults generation of warpage due to unevenness in firing shrinkingbehavior.

Moreover, it is preferable that the difference between the shrinkingstarting temperature T₂ of the second insulating sheet and the shrinkingending temperature T₃, T₄ of the first insulating sheet, the conductorgreen sheet C (T₂-T₃, T₂-T₃) is set to be 10° C. or above, especially20° C. or above.

This is because, the smaller the temperature range in which the firingshrinking actions of the different insulating sheet groups overlap witheach other, the greater the mutual shrinking inhibition effects. Notethat the firing shrinking ending temperature refers to a temperature atwhich the firing shrinking of a sheet arrives at 99% of a predeterminedfinal firing volume shrinking amount.

According to the invention, as the ceramic material of the insulatingsubstrate 11, any of an insulating substance, a dielectric substance,and a magnetic substance is usable. Moreover, in selecting at least twokinds of ceramic materials having different firing shrinking startingtemperatures, it is possible to use the ones having differentcompositions, or the ones having an identical composition but havingdifferent ceramic particle distributions or different specific surfaceareas.

The point of difference between the ceramic materials of two or morekinds is not limited to in firing shrinking starting temperature. Forexample, the ceramic materials are permitted to have different relativepermittivity, different strength, and different dielectric losscharacteristics according to objects.

Moreover, it is preferable that the ceramic material for use is firabletogether with the low-resistance conductor layer. In light of this, itis desirable to use a ceramic material which is firable at a temperatureas low as 1050° C. or below, or in particular a ceramic material whichis capable of co-firing with Ag at a temperature of 960° C. or below,especially 920° C. or below, because Ag is firable in the surroundingatmosphere. The known examples of such a low temperature-firable ceramicmaterial include: a glass powder-base material; a glass-ceramicadmixture powder-base material; and an oxide-containing admixturepowder-base material.

As the glass element, any of amorphous glass and crystallizable glass isusable. For example, the ceramic material should preferably comprise 50to 100 parts by weight of glass powder to 0 to 50 parts by weight ofceramic powder. Although there is no particular limitation, for example,the glass powder should preferably have a composition of 20 to 70 partsby weight of SiO₂, 0.5 to 30 parts by weight of Al₂O₃, and 3 to 60 partsby weight of MgO, and further, on an as needed basis, 0 to 35 parts byweight of CaO, 0 to 30 parts by weight of BaO, 0 to 30 parts by weightof SrO, 0 to 20 parts by weight of B₂O₃, 0 to 30 parts by weight of ZnO,0 to 10 parts by weight of TiO₂, 0 to 3 parts by weight of Na₂O, and 0to 5 parts by weight of Li₂O.

As the ceramic powder, at least one or more selected from the groupconsisting of Al₂O₃; SiO₂; MgTiO₃; CaZrO₃; CaTiO₃; Mg₂SiO₄; BaTi₄O₉;ZrTiO₄; SrTiO₃; BaTiO₃; and TiO₂ can be used.

The use of such a combination of glass powder and ceramic powder havingthe above-described composition makes it possible to achieve sinteringat a temperature as low as 1000° C. or below, as well as to use alow-resistance conductor element such as Cu, Ag, and Au for constitutingthe conductor layer. Moreover, by virtue of the advantage of providing alower dielectric constant, the invention is suitable for high-speedtransmission. Further, various changes can be made to the powdercomposition so long as the above-described conditions are fulfilled.This helps facilitate control and change to firing shrinking behaviors.

According to the invention, the laminated body needs to be so designedthat, at the point in time when the second insulating sheet having ahighest firing shrinking starting temperature begins to shrink in thecourse of firing, the firing shrinking of the first insulating sheet andthe conductor green sheet C is over. As stated above, such a laminatedbody can be realized by making changes to the composition of theinsulating sheet or by making changes to the grain size of the metalpowder contained in the conductor green sheet properly, for example.However, the best way is to set the volume content (%) of each of theinorganic and metal compounds contained in the green sheets to be formedinto the ceramic A, the ceramic B, and the conductor layer C at anappropriate value.

To be more specific, given that the volume content of the firstinorganic compound contained in the first insulating sheet is B₁, andthe volume content of the second inorganic compound contained in thesecond insulating sheet is B₂, then B₁/B₂ is preferably set at 0.90 orabove, especially 0.95 or above. In other words, B₁/B₂ is preferably0.90 or above, especially 0.95 or above, in which B₁ represents a volumecontent of the inorganic compound composed of the crystallizable glasspowder and the ceramic powder contained in the first insulating sheets11 a and 11 b, 12 a-(2), 12 b-(1) and B₂ represents a volume content ofthe inorganic compound composed of the crystallizable glass powder andthe ceramic powder contained in the second insulating sheets 12 a-(1)and 12 b-(2). This makes it possible to increase the temperaturedifference between T₂ and T₃, i.e., T₂-T₃, and thereby fabricate adesired laminated body with stability.

Moreover, B₃/B₂ is preferably 0.90 or above, especially 0.95 or above,in which B₃ represents a volume content of the metal compound containedin the conductor green sheet. This makes it possible to increase thetemperature difference between T₂ and T₄, i.e., T₂-T₄, and therebyfabricate a desired laminated body with stability.

Further, in the firing profile for firing the laminated body, by settingthe temperature rising speed at 15° C./min or below, further, 10° C./minor below, especially 6° C./min or below in the range of T₁ and T₂, it ispossible to fabricate a desired laminated body with higher stability.

Here, the method for manufacturing the multilayer wiring board of theinvention will be explained concretely. At the outset, the firstinsulating sheet and the second insulating sheet that exhibit differentfiring shrinking behaviors are formed as follows. A predeterminedinorganic compound, a suitable an organic binder, an organic solvent anda plasticizer, and, if necessary, a dispersant are blended, therebyforming a slurry. The slurry is molded into a tape-like component by thedoctor blade method or the like technique, and the tape is cut up toobtain green sheets of predetermined dimension.

At this time, in order for the firing shrinking ending temperature T₃ ofthe first insulating sheet to be lower than the firing shrinkingstarting temperature T₂ of the second insulating sheet, the ratio B₁/B₂(B₁ represents the volume content of the inorganic compound contained inthe first insulating sheet; and B₂ represents the volume content of theinorganic compound contained in the second insulating sheet) is set at0.90 or above. Such first and second insulating sheets cannot beobtained without determining the composition of the slurry (selection ofthe functional group of the binder, selection of the dispersant, anddetermination of the addition amount thereof) or kneading conditions inan appropriate manner.

Next, the conductor green sheet C is formed as follows. The first andsecond insulating sheets are subjected to punching or the like processto create through holes in which a conductor paste is charged. Then, asurface conductor layer, an inner conductor layer, and an electrodeconductor layer are formed by deposition, with use of a predeterminedconductor material in paste form, by means of screen printing or thelike method, and the conductor green sheet C is formed.

At this time, in order for the firing shrinking ending temperature T₄ ofthe conductor green sheet C to be lower than the firing shrinkingstarting temperature T₂ of the second insulating sheet, the B₃/B₂ ratio(B₃ represents the volume content of the metal compound contained in theconductor green sheet C; and B₂ represents the volume content of theinorganic compound contained in the second insulating sheet) is set at0.90 or above. Such a conductor green sheet C cannot be obtained withoutdetermining the composition of the slurry for forming the conductorgreen sheet C (selection of the binder, selection of the dispersant, anddetermination of the addition amount thereof) or kneading conditions inan appropriate manner.

The first and second insulating sheets thus obtained are stacked on topof one another in accordance with a predetermined lamination order,thereby forming a laminated body. The laminated body is subjected tofiring afterwards.

In a firing process, at first, when the firing temperature reaches theshrinking starting temperature T₁ of the first insulating sheet, thelaminated body is fired to shrink at a temperature rising speed of 15°C./min or below, furthermore, 10° C./min or below, especially 6° C./minor below. At this time, the second insulating sheet is still kept in anon-shrinking state and thus acts to inhibit the first insulating sheetand the conductor green sheet C from shrinking in the plane direction(X-Y direction), whereby the firing shrinking thereof is so controlledthat it occurs only in the thickness direction (Z direction). When thefiring temperature further rises and reaches the shrinking startingtemperature T₂ of the second insulating sheet, the first insulatingsheet and the conductor green sheet C in which this firing shrinking hasbeen nearly over in turn act to inhibit the second insulating sheet fromshrinking in the X-Y direction, whereby the firing shrinking thereof isso controlled that it occurs only in the Z direction. After all, in thecourse of firing, all of the first insulating sheet, the secondinsulating sheet, and the conductor green sheet C are inhibited fromshrinking in the X-Y direction and are thus caused to shrink only in theZ direction. In consequence, a resulting circuit board suffers littlefrom warpage.

Moreover, in the invention, it is desirable to ensure that thedifference in temperature between the top surface and the back surfaceof the laminated body is kept slight in the course of firing.Specifically, it will be effective to reduce the contact area betweenthe laminated body and a firing setter. To that end, the firing settermay be formed of a sintered porous body having a porosity e.g. of 30% orabove, or through holes or grooves may be drilled in the firing setter.

In general, in the laminated body, the back surface making contact witha firing jig such as a firing setter tends to be lower in temperaturethan the top surface. It occurred actually that, as compared with thefirst and second insulating sheets and the conductor green sheet Clocated near the top surface of the laminated body, those contacted by afiring setter slowed to sinter, thus delaying the time which they taketo be fired completely. In consequence, the laminated body suffered fromappreciable warpage. In order to overcome this problem, the laminatedbody-firing setter contact condition and the way of firing-settersetting need to be determined in an appropriate manner. By doing so, thedifference in temperature between the top surface and the back surfaceof the laminated body can be kept slight in the course of firing,wherefore a circuit board with little warpage can be produced.

EXAMPLES Example 1

Referring to Table 1, glass powder and ceramic powder serving as fillerwere mixed together at the mixing ratio shown in Table 1. Then,ethylcellulose (organic binder) and 2-2-4 trimethyl pentadiolmonoisobutyrate (organic solvent) are blended into the powder admixtureto form a slurry. The slurry was molded into a thin-layer component bythe doctor blade method to obtain green sheets. Whereupon, theinsulating sheets for constituting a multilayer insulating substratewere prepared for use (sheets A through H). Note that thecrystallization temperature Tc of the glass and the softening point Tgof the glass were determined, by means of DTA (Differential ThermalAnalysis), in accordance with a curve obtained with a temperature risingspeed kept at 10° C./min.

Also listed in Table 1 are the firing shrinking starting temperatures,shrinking ending temperatures, thermal expansion coefficients, anddielectric constants of the individual insulating sheets. In carryingout data measurement, the powder admixtures for forming the individualinsulating sheets were each added with wax, and then pressed at apressure of 100 MPa to form pressed powder bodies. Subsequently, thepressed powder bodies were subjected to TMA (Thermo Mechanical Analysis)in the air at temperatures falling in a range from an ambienttemperature to 1000° C., thereby measuring the shrinking startingtemperatures S, the shrinking ending temperatures E, and the thermalexpansion coefficients (as observed in a range of from an ambienttemperature to 900° C.) of the individual ceramics bodies.

Then, a through hole was drilled by punching or the like process at apredetermined position of the corresponding insulating sheet, in whichthe conductor paste containing Ag powder as shown in Table 2 is charged.Moreover, the conductor paste was screen-printed on to the surface ofthe corresponding insulating sheet to create a predetermined wiringpattern, followed by effecting a drying treatment. Whereupon, the firstand second insulating sheets were prepared for use.

In other words, the uppermost green sheet and the lowermost green sheetare defined as the first insulating sheet. The other green sheetssandwiched therebetween are defined as the second insulating sheet. Inthis regard, the crystallizable glass as shown in Table 2 wasrespectively selected so that these insulating sheets are stacked so asto constitute the laminated body shown in FIG. 1. The laminated body wasthus formed. Note that the difference between the crystallizationtemperature Tc of the glass contained in the first insulating layer andthe softening point Tg of the glass contained in the second insulatinglayer is listed in Table 2.

The laminated body was heated in the surrounding atmosphere, and thensubjected to a binder removal treatment at 400° C. Then, the temperaturewas risen to 910° C. Under this condition, a multilayer wiring boardhaving the layer arrangement shown in FIG. 1 was constructed. Note thatthe insulating layers 1 a through 1 g have a thickness of 0.1 mm, andthe multilayer wiring board has a size of 10 by 10 mm, with a thicknessof 1.0 mm.

Next, the plane direction (X-Y direction)-wise shrinking of themultilayer wiring board was measured in the following manner. Namely,the distance (length) between predetermined points of the laminated bodyin an unfired state and the distance (length) between predeterminedpoints of the multilayer wiring board in an already-fired state weremeasured for comparison. Note that 10 pieces of samples were fabricatedfor measurement, which are numbered 1 to 10. The samples were subjectedto measurement on an individual basis to obtain the mean value of allthe shrinkage data, which is regarded as the shrinkage of the multilayerboard. Moreover, in the 10 samples, the difference between the maximumshrinkage value and the minimum shrinkage value was evaluated asshrinkage variations.

Further, the presence or absence of cracks and delamination was examinedfor defect evaluation by observing the polished surface of themultilayer wiring board with use of an optical microscope.

Note that the crystallization temperature T_(c) of the crystallizableglass contained in the first insulating layer and the softening pointtemperature T_(g) of the crystallizable glass contained in the secondinsulating layer were determined, by means of DTA (Differential ThermalAnalysis), in accordance with a curve obtained with a temperature risingspeed kept at 10° C./min.

TABLE 1 GLASS POWDER CRYSTALIZATION SOFTENING FILLER SHEET INSULATINGTEMP. POINT PRECIPITATED CONTENT CONTENT NO. LAYER ° C. ° C. CRYSTALmass % KIND mass % A FIRST 785 640 C, F 70 Al₂O₃ 30 B FIRST 790 644 C,M, W 60 Al₂O₃ 40 C FIRST 810 660 C, CJ 50 Al₂O₃ 50 D FIRST 790 650 C, F,G 30 Al₂O₃ 70 E FIRST 780 639 C, F 25 Al₂O₃ 75 F SECOND 880 799 D, H 60Al₂O₃ 40 G SECOND 882 800 D, W, A 60 Al₂O₃ 40 H SECOND 885 805 D, S, M60 Al₂O₃ 40 SHRINKING SHRINKING THERMAL STARTING ENDING DIELECTRICEXPANSION SHEET TEMP. TEMP. CONSTANT COEFFICIENT NO. ° C. ° C. 2 GHz10⁻⁶/° C. A 650 740 7.6 10.0 B 656 744 8.1 9.7 C 650 740 8.9 11.1 D 648740 9.1 9.8 E 640 733 9.5 9.5 F 740 850 9.1 8.5 G 758 860 9.4 8.6 H 737840 9.4 8.7 ABBREVIATION SYMBOLS (C: CELSIAN; F: FORSTERITE; M: MULLITE;W: WILLEMITE; CJ: CORDIERITE; G: GAHNITE; D: DIOPSIDE; H: HARDESTNITE;A: ANORTHITE; S: SPINEL)

TABLE 2 FIRST SECOND INSULATING INSULATING TEMPERATURE CO-FIREDSUBSTRATE LAYER LAYER DIFFERENCE SHRINKAGE SAMPLE SHEET Tc SHEET Tg Tg −Tc SHRINKAGE VARIATIONS NO. NO. ° C. NO. ° C. ° C. % % DEFECT 1 A 785 F799 14 2.5 0.3 ABSENT 2 A 785 F 799 14 2.5 0.3 ABSENT 3 B 790 F 799 93.5 0.3 ABSENT 4 B 790 G 800 10 3 0.3 ABSENT *5 C 810 G 800 −10 10 0.7GENERATED *6 C 810 H 805 −5 8.5 0.7 ABSENT 7 D 790 G 800 10 2.5 0.3ABSENT 8 D 790 H 805 15 2 0.3 ABSENT 9 D 790 H 805 15 1.5 0.3 ABSENT 10E 780 H 805 25 0.5 0.2 ABSENT Astarisk * represents a sampleconstituting a departure from the scope of the invention.

As will be seen from the tables, in the samples Nos. 1 to 4 and 7 to 10of the multilayer wiring board of the invention, the shrinkage is below4%, and shrinkage variations are equal to 0.3% or below. Moreover,neither crack nor delamination was observed therein. It has thus beenconfirmed that the multilayer wiring board of the invention is excellentin characteristics, namely the dimensional accuracy is high, the planedirection-wise shrinkage is reduced to practically zero, shrinkagevariations are decreased, and the conductor resistance is low.

On the other hand, in the samples Nos. 5 and 6 that constitute adeparture from the scope of the invention, since the crystallizationtemperature of the crystallizable glass contained in the firstinsulating layer is higher than the softening point of thecrystallizable glass contained in the second insulating layer, itfollows that the shrinkage and shrinkage variations were found to be ashigh as 8.5% or above and 0.7% or above, respectively.

Example 2

The same as the case of Example 1, green sheets as shown in Table 1 werefabricated. Whereupon, the insulating sheets for constituting amultilayer insulating substrate were prepared for use (sheets A throughH).

Next, Ag powder composed of Ag particles of varying grain sizes wassubjected to e.g. a ball-mill mixing process to form Ag powder materialsof varying tap bulk densities. Then, an organic binder (ethylcellulose)and an organic solvent (2-2-4 trimethyl pentadiol monoisobutyrate) wereblended into the Ag powder to form a conductor paste having an Ag powdercontent of 84% by mass. The grain size and density ratio of the Agpowder used to prepare the conductor paste are listed in Table 2. Notethat the density ratio was obtained by calculation, on the basis of thetap bulk density measured in accordance with the density measurementmethod prescribed in JIS R1628 and the density (d²⁰) of Ag taken fromthe Metal Data Book compiled by The Japan Institute Of Metals, using thefollowing expression:

Density ratio=(tap bulk density of metal powder/density of metalelement)×100(%).

Then, a through hole was drilled by punching or the like process at apredetermined position of the corresponding insulating sheet, in whichthe conductor paste as shown in Table 3 is charged. Moreover, theconductor paste was screen-printed on to the surface of thecorresponding insulating sheet to create a predetermined wiring pattern,followed by effecting a drying treatment. Whereupon, the first andsecond insulating sheets were prepared for use. These insulating sheetsthus obtained were subjected to compression bonding to form a laminatedbody.

In regard to the layer arrangement of the laminated body, as shown inFIG. 1, the uppermost green sheet and the lowermost green sheet aredefined as the first insulating sheet. The other green sheets sandwichedtherebetween are defined as the second insulating sheet. Note that thedifference between the crystallization temperature Tc of the glasscontained in the first insulating layer and the softening point Tg ofthe glass contained in the second insulating layer is listed in Table 3.

The laminated body was heated in the surrounding atmosphere, and thensubjected to a binder removal treatment at 400° C. Then, the temperaturewas risen to 910° C. Under this condition, co-firing was performedthereon for 1 hour, whereupon a multilayer wiring board having the layerarrangement shown in FIG. 1 was constructed (test samples Nos. 1 to 12).In the multilayer wiring board samples, the insulating layers 1 athrough 1 g have a thickness of 0.1 mm, and the multilayer wiring boardhas a size of 50 by 40 mm, with a thickness of 0.7 mm.

In each of the multilayer wiring boards thus constructed, the ratio ofvoid in the cross section of the wiring layer and the sheet resistancewere measured. The measurement results are listed in Table 3.

The sheet resistance was measured with use of a resistance meter. Thevalues listed in Table 3 were obtained under the condition that theconductor thickness t=20 μm.

The void ratio was measured by performing SEM observation on the(mirror-) polished surface of the cross section, using the followingexpression:

(void area in wiring layer/wiring layer area)×100(%).

Note that, assuming that the void ratio in the cross section is X % andthe void ratio in the bulk is Y %, then the relationship between X and Yis given by: Y=X^(3/2).

Next, the plane (X-Y) direction-wise shrinkage of the multilayer wiringboard was measured in the following manner. Namely, the distance(length) between predetermined points of the laminated body in anunfired state and the distance (length) between predetermined points ofthe wiring board in an already-fired state were measured for comparison.With the measurement values, the shrinkage was calculated in thefollowing expression:

shrinkage: ((a−b)/a)×100(%)

wherein a represents before-firing length and b represents after-firinglength.Moreover, shrinkage variations were also measured, and the measurementresults are listed in Table 3. Note that 10 pieces of samples werefabricated for measurement, which are numbered 1 to 10. The samples weresubjected to measurement on an individual basis to obtain the mean valueof all the shrinkage data, which is regarded as the shrinkage of thewiring board. In addition, in the 10 samples, the difference between themaximum shrinkage value and the minimum shrinkage value was evaluated asshrinkage variations.

Further, the presence or absence of cracks and delamination was examinedfor defect evaluation by observing the polished surface of themultilayer wiring board with use of an optical microscope.

TABLE 3 FIRST SECOND INSULATING INSULATING TEMPERATURE LAYER LAYERDIFFERENCE SAMPLE SHEET Tc SHEET Tg Tg − Tc NO. NO. ° C. NO. ° C. ° C.11 A 785 F 799 14 12 A 785 F 799 14 13 B 790 F 799 9 14 B 790 G 800 1015 D 790 G 800 10 16 D 790 H 805 15 17 D 790 H 805 15 18 E 780 H 805 2519 D 790 G 800 10 20 D 790 G 800 10 *21  B 790 F 799 9 *22  B 790 G 80010 WIRING LAYER (Ag) CO-FIRED SUBSTRATE GRAIN DENSITY SHEET SHRINKAGESAMPLE SIZE RATIO 1) VOIDAGE RESISTANCE SHRINKAGE VARIATIONS NO. μm % %mΩ/square % % DEFECT 11 1 20 3 1.4 2.5 0.3 ABSENT 12 1 20 3 1.4 2.5 0.3ABSENT 13 1 20 3 1.4 3.5 0.3 ABSENT 14 1 20 3 1.4 3 0.3 ABSENT 15 1 35 21.4 2.5 0.3 ABSENT 16 1 35 2 1.4 2 0.3 ABSENT 17 1 35 2 1.4 1.5 0.3ABSENT 18 1 35 2 1.4 0.5 0.2 ABSENT 19 1 15 4 1.4 2 0.3 ABSENT 20 1 15 41.4 2 0.3 ABSENT *21  3 12 7 1.6 3.5 0.3 ABSENT *22  3 10 10  1.7 3 0.3ABSENT Asterisk * represents a sample constituting a departure from thescope of the invention. 1) Density ratio = (tap bulk density of metalpowder/density of metal element) × 100

As will be seen from the tables, in the samples Nos. 11 to 20 of themultilayer wiring board of the invention, the shrinkage is below 4%,shrinkage variations are equal to 0.3% or below, and the sheetresistance is below 1.5 mΩ/square. Moreover, neither crack nordelamination was observed therein. It has thus been confirmed that themultilayer wiring board of the invention is excellent incharacteristics, namely the dimensional accuracy is high, the planedirection-wise shrinkage is reduced to practically zero, shrinkagevariations are decreased, and the conductor resistance is low.

On the other hand, in the samples Nos. 21 and 22 constituting adeparture from the scope of the invention in which the void ratio in thewiring layer exceeds 5%, the sheet resistance takes on a value as highas 1.6 mΩ/square or above.

Example 3

A multilayer wiring board having such a constitution as shown in FIG. 2was fabricated in the following manner. At the outset, two differentceramic materials a and b were prepared for use. The ceramic material aconsists of 80% by weight ofSiO₂—Al₂O₃—MgO—CaO—BaO—B₂O₃—ZnO—TiO₂—Na₂O—Li₂O glass powder and 20% byweight of Al₂O₃ powder having an average particle diameter ofapproximately 1 μm. The ceramic material B consists of 60% by weight ofSiO₂—Al₂O₃—MgO—CaO—BaO—SrO—B₂O₃ glass powder and 40% by weight of Al₂O₃powder having an average particle diameter of approximately 1 μm. Then,suitable binder and dispersant were kneadingly blended into each of theceramic materials A and b to form a slurry. The slurry was processedinto first and second insulating sheets by the doctor blade method,whereupon ceramic green sheets having different inorganic-compoundvolume content as shown in Table 4 were obtained.

The lamination order of the ceramic green sheets conforms to that shownin FIG. 2. Next, a through hole was drilled in each of the green sheets,in which a conductor paste containing Ag powder is charged. Then, asurface conductor layer, an inner conductor layer, and a back-surfaceconductor layer were printed on to the corresponding surfaces of theceramic green sheets with use of a conductor paste. As the mainconductor material of the surface conductor layer, the inner conductorlayer, and the back-surface conductor layer, silver powder was used.Suitable organic vehicle and surface-active agent were blended into thesilver powder, and the admixture was kneaded by a three-roll mill untilthe agglomeration of the silver powder particles was eliminated, therebyforming the conductor paste. In this way, conductor green sheets havingdifferent metal-compound volume content as shown in Table 4 wereobtained.

After undergoing positional alignment, the green sheets were stackedtogether to form a laminated body. The laminated body was subjected to abinder removal treatment at 400° C. in the surrounding atmosphere, andwas thereafter heated at 910° C. in the surrounding atmosphere,whereupon the multilayer wiring board was fabricated.

As the firing setter, a porous material composed predominantly ofAl₂O₃—SiO₂ was used. At this time, several types of firing setters wereprepared for use: the one having a thickness of 3 mm with a through holeof approximately 1 mm in diameter; the one having a thickness of 3 mmwithout through hole; and the one having a thickness of 1 mm withoutthrough hole. Moreover, a spacer was disposed between the bottom surfaceof the firing setter and a firing furnace to change the way offiring-setter setting, so that the firing setter may be heated from itsbottom surface side.

Here, the thicknesses of the first and second insulating sheets were setat 50 μm and 100 μm, respectively. In order to determine the volumecontent of the inorganic compound in each of the green sheets, at first,the density of the raw green sheet (g/cm³) was measured. Then,calculation is made on the basis of the measured value, the density ofthe inorganic compound, the density of the additive contained in theslurry, and the composition of the green sheet in itself.

In order to examine the firing shrinking behaviors of the firstinsulating sheet, the second insulating sheet, and the conductor greensheet c, predetermined holes were drilled in the individual green sheetsfor dimension evaluation. Then, in the course of firing, the greensheets were taken out from the firing furnace to check the intervalbetween the holes. On the basis of the dimensional change, the shrinkageof each of the green sheets was calculated, whereby the shrinkingstarting and ending temperatures were determined. For measurement wasused the conductor green sheet C having a thickness of 100 μm which wasobtained by a plural repetition of printing and drying of the conductorpaste.

A multilayer wiring board sample in finished form (according to themultilayer wiring board as shown in FIG. 2) was subjected to planedirection-wise warpage evaluation. Specifically, irregularities existingnear the 7 mm square electrode of the surface layer conductor weremeasured by using a three dimensional measuring machine. Then, thedifference between the maximum asperity value and the minimum asperityvalue was determined as a warpage. The measurement results are listed inTable 4.

TABLE 4 INORGANIC COMPOUND VOLUME CONTENT IN GREEN SHEET (vol %) FIRSTSECOND INSULATING INSULATING CONDUCTOR FIRING PROFILE SAMPLE SHEET SHEETLAYER TEMPERATURE RISING NO. B₁ B₂ B₃ B₁/B₂ B₃/B₂ SPEED (° C./MIN)FIRING SETTER *23  45 52 42 0.87 0.81 10 3 mm/WITHOUT HOLE *24  47 52 420.90 0.81 10 3 mm/WITHOUT HOLE 25 49 52 47 0.94 0.90 10 3 mm/WITHOUTHOLE 26 56 52 47 1.08 0.90 10 3 mm/WITHOUT HOLE 27 51 52 50 0.98 0.96 103 mm/WITHOUT HOLE 28 51 48 54 1.06 1.13 10 3 mm/WITHOUT HOLE 29 52 57 520.91 0.91 15 3 mm/WITHOUT HOLE 30 52 57 52 0.91 0.91  6 3 mm/WITHOUTHOLE 31 51 52 50 0.98 0.96 10 1 mm/WITHOUT HOLE 32 51 52 50 0.98 0.96 103 mm/WITH HOLE 33 51 52 50 0.98 0.96 10 3 mm/WITH SPACER SECONDINSULATING FIRST INSULATING SHEET SHEET CONDUCTOR SHRINKING SHRINKINGSHRINKING LAYER STARTING ENDING STARTING SHRINKING RELATION WITH T₂SAMPLE TEMP. TEMP. TEMP. ENDING TEMP. T₃ − T₂ T₄ − T₂ WARPAGE NO. T₁ (°C.) T₃ (° C.) T₂ (° C.) T₄ (° C.) ° C. ° C. μm *23  660 755 745 780 −10−35 75 *24  640 730 745 780 15 −35 53 25 630 720 745 740 25 5 24 26 600710 745 740 35 5 26 27 620 720 745 720 25 25 20 28 620 720 755 715 35 4018 29 620 725 732 730 7 2 34 30 620 710 725 710 15 15 22 31 620 720 745720 25 25 10 32 620 720 745 720 25 25  9 33 620 720 745 720 25 25 12

As will be understood from Table 4, the following facts have beenconfirmed. In the samples Nos. 25 to 33 embodying the invention thatexhibit a B₁/B₂ ratio of 0.90 or above and a B₃/B₂ ratio of 0.90 orabove, and also fulfill the condition of T₃<T₂, T₄<T₂, the degree ofwarpage is less than 35 μm. By way of contrast, in the samples Nos. 23and 24 constituting a departure from the scope of the invention thatexhibit a B₁/B₂ ratio of 0.90 or below and a B₃/B₂ ratio of less than0.90, and also fulfill the condition of T₃>T₂, T₄>T₂, the degree ofwarpage exceeds 50 μm, which is larger than the degree of warpage in thecase of those embodying the invention. Moreover, in performing firing,by setting the temperature rising speed at 10° C./min. or below in therange of from T₁ to T₂, it is possible to increase the difference intemperature between T₂ and T₃, as well as T₂ and T₄, and therebydecrease the degree of warpage. Further, the difference in temperaturebetween the top surface and the back surface of the sample can be keptslight by making the firing setter smaller in thickness, by drilling athrough hole therein, or by providing a spacer, wherefore the degree ofwarpage can be decreased.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

1-7. (canceled)
 8. A method for manufacturing an insulating substrate,comprising the steps of: preparing a laminated body composed of firstand second insulating sheets containing crystallizable glass powder andceramic powder, wherein a crystallization temperature of thecrystallizable glass powder contained in the first insulating sheet islower than a softening point of the crystallizable glass powdercontained in the second insulating sheet; and carrying out co-firing ofthe laminated body to fabricate an insulating substrate.
 9. The methodof claim 8, wherein the first and second insulating sheets each have acrystallizable glass powder content of 30% by mass or above.
 10. Themethod of claim 8, wherein the crystallizable glass contained in thefirst and second insulating sheets forms at least one selected from thegroup consisting of diopside, hardestnite, celsian, cordierite,anorthite, gahnite, willemite, spinel, mullite, forsterite, and suanite.11. The method of claim 8, wherein the crystallizable glass powdercontained in the first insulating sheet comprises: 5 to 20% by mass ofSiO₂; 40 to 50% by mass of MgO; 10 to 30% by mass of B₂O₃; and 0 to 30%by mass of at least one selected from the group consisting of CaO,Al₂O₃, SrO, ZnO, TiO₂, Na₂O, BaO, SnO₂, P₂O₅, ZrO₂, and Li₂O, whereasthe crystallizable glass powder contained in the second insulating sheetcomprises: 30 to 50% by mass of SiO₂; 10 to 25% by mass of MgO; and 25to 55% by mass of at least one selected from the group consisting ofB₂O₃, CaO, Al₂O₃, SrO, ZnO, TiO₂, Na₂O, SnO₂, P₂O₅, ZrO₂, and Li₂O. 12.A method for manufacturing a multilayer wiring board comprising thesteps of: preparing a plurality of the first and second insulatingsheets of claim 8; applying a conductor paste in a wiring pattern atleast to surfaces of partial insulating sheets among a plurality of theinsulating sheets to form a conductor layer, the conductor layerpredominantly containing at least one selected from the group consistingof Au, Ag, Cu, Pd, and Pt; fabricating a laminated body by stacking aplurality of the insulating sheets; and sintering the laminated body.13. The method of claim 12, wherein the conductor paste is prepared byblending an organic binder and a solvent into metal powder having a tapbulk density of 15% or above based on a density (d²⁰) of a metalelement.
 14. The method of claim 12, wherein the second insulating sheethas a firing shrinking starting temperature T₂ which is higher than botha shrinking ending temperature T₃ of the first insulating sheet and ashrinking ending temperature T₄ of the conductor layer.
 15. The methodof claim 14, wherein B₁/B₂ is 0.90 or above, in which B₁ represents avolume content of the inorganic compound composed of the crystallizableglass powder and the ceramic powder contained in the first insulatingsheet, and B₂ represents a volume content of the inorganic compoundcomposed of the crystallizable glass powder and the ceramic powdercontained in the second insulating sheet.
 16. The method of claim 15,wherein B₃/B₂ is 0.90 or above, in which B₃ represents a volume contentof the metal compound contained in the conductor layer.